Four infinitely long parallel wires pass through the four corners of a square PQRS of side a that lies in a plane perpendicular to the wires (see figure). They all carry equal steady currents I . Currents in the wires passing through P and Q point out of the page, and the currents in the wires passing through R and S point into the page as shown in the figure. The magnitude of the magnetic field at the centre O of th